1. Field of the Invention
The present invention relates to a liquid crystal device (LCD). Such LCDs may be used in reflective or transmissive configurations.
2. Description of the Related Art
FIG. 1 of the accompanying drawings illustrates a known type of reflective LCD having a passive matrix addressing arrangement. The device comprises mirror electrodes 1 in the form of parallel stripes extending vertically in FIG. 1. The mirror electrodes 1 are disposed above a rear light absorber 2 which is visible in the view from above of FIG. 1 in the gaps between the mirror electrodes 1. Transparent electrodes, such as indium tin oxide (ITO) electrodes 3 are disposed above the liquid crystal layer and take the form of elongate parallel electrodes which extend horizontally in FIG. 1. The gaps between the electrodes 3 result in unaddressed regions 4 which are disposed above the reflective mirror electrodes 1. Thus, light passing through the regions 4 is reflected and is not absorbed by the rear absorber 2.
The LCD shown in FIG. 1 is of the ferroelectric liquid crystal (FLC) type in which the liquid crystal layer acts as a retarder with in-plane optic axes switching. A fixed quarter wave plate with its optic axis oriented at 75xc2x0 is disposed between the FLC layer and the mirror electrodes. Picture elements (pixels) in the white state, such as 5, have their optic axes switched to an angle of xe2x88x927.5xc2x0 whereas pixels such as 6 in the black state have their optic axes switched to an angle of +15xc2x0. The unaddressed regions 4 are not subjected to an applied addressing field between the electrodes 1 and the electrodes 3 and thus adopt an undefined state. This state corresponds to a non-black state so that incident light on the regions 4 is at least partly reflected and is visible to a viewer of the display. This results in a reduction of the contrast ratio.
A known technique for avoiding this problem is disclosed in D. G. McDonnell et al, xe2x80x9cAn Ultra-High-Resolution Ferroelectric Liquid-Crystal Video Displayxe2x80x9d, Digest of Technical Papers, Society for Information Display International Symposium, 1993, p 654. According to this technique, the liquid crystal in the inter-pixel gaps or regions such as 4 in FIG. 1 to switched by fringing fields into a controlled state. However, it is not always possible to achieve such switching, for example because of limitations in addressing, device configuration and material characteristics or because the inter-pixel gap contains a non-switchable spacing element.
In cases where it is not possible to switch the inter-pixel gaps, it is known to provide a black matrix for masking such regions, for example as disclosed in Koden et al, xe2x80x9cKey Technologies for the xcfx84-Vmin Mode FLCDxe2x80x9d, IDW 1997, p 269 and in D. E. Castleberry et al, xe2x80x9cA One Mega-Pixel Colour a-Si TFT Liquid-Crystal Displayxe2x80x9d, Digest of Technical Papers, Society for Information Display International Symposium, 1988, p 232. However, this increases the processing steps and hence the cost of making such displays and decreases the fill factor of the display.
It is also known to provide cell spacing by means of patterned structures of uniform height but this also reduces the contrast ratio of LCDs. For example, Koden et al (as mentioned above) describes the use of continuous spacer walls to achieve mechanical stability in smectic devices and Colgan et al xe2x80x9cOn-chip metallisation layers for reflective light valvesxe2x80x9d, IBM Journal of Research and Development, vol. 42 no. 3 1998 discloses the use of silicon dioxide spacer posts at the corners of pixels. FIG. 2 of the accompanying drawings illustrates the arrangement of such known devices with spacer walls 7 disposed in the gaps between the upper electrodes 3. The spacers 7 are transparent and are made of substantially isotropic material. Incident light is therefore reflected by the underlying portions of the mirror electrodes 1 and this again reduces the contrast ratio of such LCDs.
It Is known for the spacer walls 7 to be disposed between the gaps in the mirror electrodes 1 as illustrated in FIGS. 3 and 4 of the accompanying drawings. However, any Inaccuracies in positioning and size of the spacer walls 7 results in a reduction of contrast ratio. For example, as shown in FIG. 3 at 8, the spacer walls 7 are misaligned with the gaps between the mirror electrodes 1 and are therefore skewed so as to overlap the partially reflective regions of the pixels. Translational errors may also occur and result in the spacer wall 7 overlapping the partially reflective regions. As shown in FIG. 4 at 9, if the spacer walls 7 are too wide, they will again overlap the mirror electrodes 1. Light passing through the spacer walls 7 and striking the overlapping portions of the mirror electrodes 1 is reflected back out of the LCD and results in a reduction in contrast ratio.
The spacers 7 may be made of a black polymer material, for example as disclosed in D. E. Castleberry et al (as mentioned above) and in C. M. Healer et al. xe2x80x9cPigment-Dispersed Organic Black-matrix Photoresists For LCD Colour Filtersxe2x80x9d, SID 1995 Digest, p 446. However, in order to avoid compromising the contrast ratio of the LCD, such a material would have to have sufficient light-absorbing properties and, in practice, there is some reduction in contrast ratio. Also, the spacers are disposed within the liquid crystal layer and the dyes or pigments used in such materials can contaminate the liquid crystal. This compromises the alignment quality and switching behaviour of the LCD.
The presence of the spacers 7 in the liquid crystal layer can also cause xe2x80x9cpinningxe2x80x9d of the liquid crystal molecules adjacent the spacers so that the liquid crystal adjacent the spacers is not substantially affected by the applied addressing field. As described hereinafter, the liquid crystal may be pinned in the black or white state and this reduces contrast ratio and aperture ratio of the device. It is thus necessary to provide a black matrix which is wider than the spacers 7 so as to mask such pinned white states. This increases the number of production process steps and hence the cost of such devices while substantially decreasing the fill factor of such devices.
According to the invention, there is provided a liquid crystal device comprising a liquid crystal layer containing a plurality of pixels separated by inter-pixel gaps, each of the pixels having a first optical state resulting in maximum light attenuation, characterised by at least one spacer disposed in the inter-pixel gaps and having substantially the same optical property as the first pixel optical state.
The at least one spacer may comprise a plurality of pillars.
The at least one spacer may comprise a plurality of walls. The walls may be continuous. The walls may enclose the pixels. The wells may fill the inter-pixel gaps.
The optical property may comprise changing the polarisation of light. The optical property may comprise retardation with a predetermined optic axis orientation. The device may comprise a linear polariser for transmitting light with a first direction of linear polarisation, a reflector, a half waveplate disposed between the polariser and the reflector and a quarter waveplate disposed between the half waveplate and the reflector, the liquid crystal layer comprising one or both of the half waveplate and the quarter waveplate.
The liquid crystal layer may be of the in-plane switching type and the at least one spacer may have the same retardation as the liquid crystal layer and an optic axis oriented in the same direction as the liquid crystal layer in the first optical state. The liquid crystal of the layer may be a smectic liquid crystal. The liquid crystal may be a ferroelectric liquid crystal.
The liquid crystal layer may comprise the half waveplate whose optic axis is switchable between xe2x88x927.5xc2x0 and +15xc2x0 to the first direction, the at least one spacer may have an optic axis at +15xc2x0 to the first direction, and the quarter waveplate may have an optic axis at +75xc2x0 to the first direction.
The liquid crystal layer may comprise the quarter waveplate whose optic axis is switchable between 75xc2x0 and 120xc2x0 to the first direction, the at least one spacer may have an optic axis at 75xc2x0 to the first direction, and the half waveplate may have an optic axis at 15xc2x0 to the first direction.
The liquid crystal layer may comprise the half waveplate whose optic axis is switchable between 0xc2x0 and 15xc2x0 to the first: direction, the at least one spacer may have an optic axis at 15xc2x0 to the first direction and the quarter waveplate may comprise a further liquid crystal layer whose optic axis is switchable between 75xc2x0 and 90xc2x0 to the first direction and at least one further spacer having an optic axis at 75xc2x0 to the first direction.
The liquid crystal of the layer may be a nematic liquid crystal. The liquid crystal layer may be a bistable twisted nematic liquid crystal layer.
The liquid crystal layer may comprise the quarter waveplate which is switchable between a 0xc2x0 twist state with an optic axis at 75xc2x0 to the first direction and a 360xc2x0 twist state, the at least one spacer may have an optic axis at 75xc2x0 to the first direction and the half waveplate may have an optic axis at 15xc2x0 to the first direction.
The liquid crystal layer may comprise the quarter waveplate which is switchable between a 0xc2x0 twist state with an optic axis at 100xc2x0 to the first direction and a 360xc2x0 twist state, the at least one spacer may have an optic axis at 100xc2x0 to the first direction, the half waveplate may have an optic axis at xcex1xc2x0 to the first direction, and a further half waveplate having an optic axis at 5xcex1xc2x0 to the first direction may be disposed between the half waveplate and the liquid crystal layer. xcex1 may be equal to 6.9xc2x0.
The device may comprise a linear polariser for transmitting light with a first direction of linear polarisation, a reflector, and a retarder having a retardation of substantially 208 nm and an optic axis at 14xc2x0 to the first direction, the liquid crystal layer being disposed between the retarder and the reflector and being switchable between a 67xc2x0 twist state, having a retardation of substantially 136 nm and an optic axis at a surface of liquid crystal layer nearer the retarder at 29xc2x0 to the first direction, and a xe2x88x92293xc2x0 or +427xc2x0 twist state, the at least one spacer having an optic axis at a surface thereof nearer the retarder at 29xc2x0 to the first direction and a 67xc2x00 twist state.
The at least one spacer may have an optical anisotropy which is greater than that of the liquid crystal layer and may be provided with an optically isotropic further spacer such that the combined thickness of the at least one spacer and the further spacer is substantially equal to the thickness of the liquid crystal layer.
The liquid crystal layer may have an optical anisotropy which is greater than that of the at least one spacer and may be provided with an optically isotropic further spacer such that the combined thickness of the liquid crystal layer and the further spacer is substantially equal to the thickness of the at least one spacer.
The at least one spacer may comprise a cured reactive mesogen.
The at least one spacer may be arranged to bias the liquid crystal of the layer in contact therewith to the first optical state. The at least one spacer may be optically anisotropic and may have a molecular alignment corresponding to the molecular alignment of the liquid crystal when in the first optical state.
The device may comprise a passive matrix addressing arrangement including a set of stripe electrodes constituting a or the reflector.
It is thus possible to provide a device in which one or more spacers have the same optical property as the xe2x80x9cblackxe2x80x9d liquid crystal state. The presence of such a spacer or spacers results in little or no loss of contrast ratio in such devices. For example, such spacers may be disposed above reflective regions of a reflective display, for example as illustrated in FIGS. 1 and 5 of the accompanying drawings, so as substantially to prevent reflection from the underlying reflector.
Such spacers may also be arranged to bias the xe2x80x9cblackxe2x80x9d state of the liquid crystal adjacent the spacers. This eliminates or reduces the problem of white state pinning as described hereinbefore and thus avoids substantial reduction in contrast ratio or the need to provide a larger black matrix area.